Ef­fects of soil qual­ity and seed health on yields

This ar­ti­cle sum­marises the re­sults of the sec­ond year of a three­year project funded by the Sus­tain­able Farm­ing Fund (SFF) study­ing the ef­fects of soil qual­ity and seed health on potato yields.


The project fo­cused on soil struc­ture and ro­ta­tion his­tory for 15 sites in Can­ter­bury. There were four­plot tri­als at each site (Rus­set Bur­bank and In­no­va­tor, treated with for­ma­lin and un­treated).


• The ma­jor in­flu­ence on yield was soil qual­ity, and both seed-borne and soil-borne dis­ease had lit­tle im­pact.

• The crop his­tory score com­bined with the score fac­tor for soil struc­tural con­di­tion ex­plained 39% of the yield vari­a­tion for In­no­va­tor and 52% for Rus­set Bur­bank. If soil qual­ity is poor then grow­ers should

con­sider grow­ing In­no­va­tor in pref­er­ence to Rus­set Bur­bank.

• There was a good cor­re­la­tion be­tween yield and a 10-year crop his­tory score, and be­tween yield and a one-off soil struc­tural con­di­tion score, show­ing that these two in­de­pen­dent meth­ods could be use­ful for gaug­ing pad­dock suit­abil­ity for grow­ing pota­toes.

• More grass in a ten-year his­tory im­proved soil re­silience and en­hanced root­ing hos­pi­tal­ity for pota­toes, thus en­abling the crop to ac­cess more re­sources. For Rus­set Bur­bank, this equated to an av­er­age 3.5 t/ha lift in yield for ev­ery year in the pre­vi­ous ten-year his­tory a field was in grass.

• For­ma­lin dip­ping did not sig­nif­i­cantly con­trol seed-borne or soil-borne dis­eases in the glasshouse or the field.

• Seed could have trans­ferred to the field Rhi­zoc­to­nia solani, caus­ing stem canker, and Spon­gospora sub­ter­ranea, caus­ing root galls, as all glasshouse plants were in­fected with these dis­eases.

• Stem canker and root gall in­ci­dence and sever­ity was greater if the pad­dock his­tory in­cluded a pre­vi­ous potato crop, com­pared with no potato his­tory, but yield was un­af­fected.

• Dis­ease sever­ity was higher in fields with pre­dom­i­nantly grass his­to­ries, com­pared with mainly crop­ping his­to­ries. How­ever, gross yield was greater from pad­docks in which grass had been grown (86 t/ha) than from crop fields (75 t/ha).


In year one of this three-year Sus­tain­able Farm­ing Fund project a sur­vey of 18 potato crops in Pukekohe, Manawatu and Can­ter­bury in­di­cated that soil com­paction and the pres­ence of soil-borne and seed-borne pathogens were likely to be the main fac­tors lim­it­ing yield.

Year two of the project fo­cused on defin­ing the im­pact of seed health and soil qual­ity on potato crop per­for­mance. This re­quired care­ful man­age­ment of po­ten­tial sources of vari­abil­ity (cul­ti­var, soil type, cli­mate and crop man­age­ment). This was achieved by host­ing all tri­als in one re­gion (Can­ter­bury) and plant­ing the same seed lines in four plots (Rus­set Bur­bank and In­no­va­tor, treated and un­treated with for­ma­lin) in each of 15 potato fields. The 15 fields were grouped into four field cat­e­gories re­lated to pad­dock his­tory and soil health:

1. Dis­eased – pre­vi­ous potato crops within the last 10 years, ‘good’ soil struc­ture – at least 5 years of grass in the 10-year his­tory (1 field).

2. Dis­eased – pre­vi­ous potato crops within the last 10 years, ‘poor’ soil struc­ture – at least 5 years of arable crops in the 10-year his­tory (6 fields).

3. Clean – no pre­vi­ous potato crops within the last 10 years, ‘good’ soil struc­ture – at least 5 years of grass in the 10year his­tory (4 fields).

4. ‘Clean’ – no pre­vi­ous potato crops within the last 10 years, ‘poor’ soil struc­ture – at least 5 years of arable crops in the 10-year his­tory (4 fields). Crop his­to­ries were col­lated for a 10year pe­riod (2005/06 to 2015/16) for each field, and a crop score ap­plied to each main an­nual crop, de­pend­ing on its abil­ity to help main­tain or re­store soil struc­ture (fal­low = 0, 1 = weak rooted crop e.g. onions, 4 = strong­est root­ing crop e.g. grass. Max­i­mum score = 40). The sum of the 10 crop scores made up the crop his­tory score. Potato plant health in each plot was mon­i­tored four times dur­ing crop growth, with a soil ag­gre­gate sta­bil­ity score (test­ing soil im­pact re­silience) and soil struc­tural con­di­tion score (a vis­ual test for root hos­pi­tal­ity) mea­sured once in mid­sea­son, and fi­nal yield mea­sured at har­vest.

Whole seed from the same Rus­set Bur­bank and In­no­va­tor seed lines, ei­ther dipped or undipped in for­ma­lin, were grown out in pot­ting mix (low dis­ease risk) in a glasshouse to check for the pres­ence of vi­able seed-borne dis­eases. The tem­per­a­ture in the glasshouse was set at 16 C, op­ti­mal for soil-borne dis­ease de­vel­op­ment, and there were 10 sin­gle plant repli­cates. No dis­eases were vis­i­ble on the tu­bers at plant­ing.


Seed and soil health

Com­mer­cial for­ma­lin dip­ping had lit­tle ef­fect on con­trol­ling seed-borne dis­ease in the glasshouse plants, as all plants de­vel­oped symp­toms of Rhi­zoc­to­nia stem canker and Spon­gospora root galls. How­ever, Rus­set Bur­bank seed was less dis­eased than In­no­va­tor, and for­ma­lin slightly re­duced stem canker sever­ity, al­though not sig­nif­i­cantly.

For­ma­lin dip­ping did not re­duce the in­ci­dence or sever­ity of the two dis­eases in the field tri­als. This meant that it was not pos­si­ble to com­plete one of the ob­jec­tives of the trial, to de­fine the rel­a­tive con­tri­bu­tion of seed-borne and soil-borne dis­ease to the in­ci­dence and sever­ity of dis­ease in the field. How­ever, the com­bined ef­fect of any seed-borne and soil­borne pathogens af­fected dis­ease ex­pres­sion dif­fer­ently for crops in the var­i­ous field cat­e­gories. >

The risk of stem canker in­ci­dence in­creased from 70% to 83% when more than five years of grass was in­cluded in the 10-year pad­dock his­tory. The risk of Spon­gospora dis­eases in­creased from 24% to 73% where pota­toes had been grown once be­fore, and in­creased from 3% in pad­docks with a mainly crop his­tory, to 46% for pad­docks with a mainly grass his­tory (Ta­ble 1).

Soil phys­i­cal qual­ity

Soil from most fields with a long term grass his­tory, i.e. a crop his­tory score of greater than 28 and at least seven years in grass, had a higher soil ag­gre­gate sta­bil­ity (range 1.8 to 2.2mm Mean Weight Di­am­e­ter (MWD)). Based on a Plant & Food Re­search study of 105 arable crops, these lev­els were over the thresh­old of 1.5mm MWD needed to grow crops that are likely to at least equal the re­gional av­er­age yield (Fig­ure 1a).

The soil struc­tural con­di­tion score was closely as­so­ci­ated with the crop his­tory score (Fig­ure 1b). This shows that much of the im­prove­ment in the abil­ity of the soil to pro­vide an ad­e­quate en­vi­ron­ment for op­ti­mum potato root growth was due to the long > 7 years grass term grass his­tory. This was even af­ter the in­ten­sive cul­ti­va­tion used to plant pota­toes, when the soil con­di­tion score mea­sure­ments were taken.

Greater val­ues of ag­gre­gate sta­bil­ity, soil struc­tural con­di­tion score and crop his­tory score all in­di­cate im­proved po­ten­tial root hos­pi­tal­ity.

There was a strong cor­re­la­tion (P = 0.012 for In­no­va­tor and P = 0.002 for Rus­set Bur­bank) be­tween gross yield and a fac­to­rial of crop his­tory score and soil struc­tural con­di­tion score. When com­bined, they helped to de­scribe the in­flu­ence of soil qual­ity on yield (Fig­ure 2). For In­no­va­tor, about 39% of the yield vari­a­tion was ex­plained by the phys­i­cal state of the soil; whereas Rus­set Bur­bank was more sen­si­tive to poorly struc­tured soil, with yield in­creas­ing more strongly in re­sponse to im­proved soil struc­ture (52% yield

vari­a­tion ex­plained). For Rus­set Bur­bank, this trans­lated into an ex­tra 3.5 t/ha yield for ev­ery year a pad­dock was in grass dur­ing the pre­vi­ous 10 years.

Potato yield

For mar­ketable yield, In­no­va­tor yielded 81 t/ha, (P = <0.001), 14t/ha more than Rus­set Bur­bank (67 t/ha). Ir­re­spec­tive of cul­ti­var, pota­toes grown in fields that were pre­vi­ously in grass yielded more (79 t/ha, P = 0.024) than those grown in crop fields (69 t/ha). Yield was un­af­fected by for­ma­lin treat­ment and whether or not pota­toes were one of the crops in the crop­ping his­tory.


This Year two re­search aimed to de­ter­mine the in­flu­ence of crop his­tory, soil qual­ity and soil-borne and seed-borne dis­ease on potato yield. Re­sults in­di­cated that im­prove­ments in soil struc­ture re­sult­ing from a grass-dom­i­nant his­tory, were syn­ony­mous with higher yields. This was de­spite that fact that soil-borne dis­ease in­ci­dence was higher in the fields that had been in grass. This in­di­cates that more em­pha­sis could be placed on scru­ti­n­is­ing crop­ping his­tory and soil struc­tural qual­ity be­fore se­lect­ing a par­tic­u­lar field for grow­ing pota­toes.

Dis­ease risk also in­creased in pad­docks where pota­toes had been grown in the last 10 years, but this fac­tor did not re­sult in re­duced yield. For­ma­lin dip­ping of seed did not as­sist with seed-borne dis­ease con­trol, and all seed used in the ex­per­i­ment had a high in­ci­dence of dis­ease present. Fur­ther in­ves­ti­ga­tion is needed to de­ter­mine how seed health may be lim­it­ing yield po­ten­tial.

In the fi­nal year of this project, we hope to ex­plore the link be­tween crop his­tory, phys­i­cal soil qual­ity and potato yield for a wider range of crops in ma­jor potato grow­ing re­gions. Ex­ten­sion will also be a ma­jor fo­cus. We will look at de­vel­op­ing or re­fin­ing field soil tests and/or cal­cu­la­tors or apps, along with in­for­ma­tion pack­ages to quickly in­form a grower of the phys­i­cal state of a pad­dock prior to sow­ing the crop.


This re­search is con­ducted by the Foun­da­tion for Arable Re­search (FAR) for Pota­toes New Zealand Inc. Thanks to all the grow­ers in­volved who pro­vided land and aided in plant­ing, to the Plant & Food Re­search and FAR teams for all as­sess­ments and to the in­dus­try for help in col­lect­ing seed and for aid at har­vest time.

▴ Fig­ure 1a The re­la­tion­ship be­tween crop his­tory score and ag­gre­gate sta­bil­ity. The greater the R2 value the stronger the re­la­tion­ship be­tween the two vari­ables. The red line is the ag­gre­gate sta­bil­ity value be­low which crops are likely to yield be­low the re­gional av­er­age.

▴ Fig­ure 1b The re­la­tion­ship be­tween crop his­tory score and soil struc­tural con­di­tion score.

▴ Ta­ble 1. Chance (%) of dis­ease oc­cur­ring for the dis­eases Rhi­zoc­to­nia stem canker and Spon­gospora root galls, un­der con­trast­ing crop­ping his­to­ries av­er­aged for all 15 sites

◀ Fig­ure 2 The re­la­tion­ship be­tween a fac­to­rial of crop his­tory score and soil struc­tural con­di­tion score and gross yield. The greater the R2 value the more of the yield vari­a­tion that can be ex­plained by the phys­i­cal state of the soil.

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